1. Field of the Invention
The present invention relates to a cathode for an electron gun used in a cathode ray tube, and more particularly, it relates to a cathode for an electron gun for increasing its life cycle under a high current density load by ensuring a diffusion route of reducing element steadily, used for generating free radical barium atom.
2. Description of the Prior Art
A cathode ray tube is a device for forming an image by excitation light emission of a fluorescent material of a screen by landing an electron emitted from an electron gun and accelerated by high voltage on the fluorescent material.
FIG. 6 is a general structural diagram of a cathode for an electron gun in a cathode ray tube. In FIG. 6, the cathode comprises a heater 4 in a sleeve 2, a cap-formed base metal 6 mainly composed of nickel Ni and containing a small amount of reducing elements such as silicone Si and magnesium Mg on the upper side of sleeve 2, and an electron emitting material layer 8 mainly composed of alkaline earth metal oxide containing barium at least on the cap-formed base metal 6.
In such a cathode, the metal oxide and the reducing element react to each other by heat generated from the heater to generate free radical barium atom, and thereafter thermion is emitted by using free radical barium.
An electron emission capacity of the cathode for the electron gun is influenced by a supply amount of free radical barium contained in the metal oxide.
However, since the cathode ray tube has a tendency of enlargement and high precision recently, a cathode which can supply free radical barium atom for a long time in high current density is required.
In Korean patent laid-open No. 96-15634, a cathode restraining free radical barium atom from evaporating by adding both of lanthanum La compound and magnesium compound Mg or La--Mg mixed compound to the electron emitting material layer containing alkaline earth metal oxide is disclosed.
However, in the conventional cathode, an intermediate layer 10 is generated in a boundary between the base metal 6 and the electron emitting material layer 8 by reaction as shown in FIG. 7, and it results to shorten the life of the cathode under high current density load of 2.about.3 A/cm.sup.2.
The intermediate layer 10 is generated by reaction of barium oxide pyrolized from barium carbonate and silicone or magnesium.
[Reaction formula 1] EQU BaO+Mg.fwdarw.MgO+Ba.uparw.
[Reaction formula 2] EQU 4BaO+Si.fwdarw.Ba.sub.2 SiO.sub.4 +2Ba.uparw.
Free radical barium atom generated by the reaction formula 1 or 2 is served to emit electron, however, MgO or Ba.sub.2 SiO.sub.4 is additionally generated by the same reaction formulas to generate the intermediate layer 10 in the boundary between the base metal 6 and the electron emitting material layer 8.
Such an intermediate layer 10 interferes the reaction for generating free radical barium atom requiring the reducing agent by obstructing diffusion of the reducing agent contained in the base metal 6, to shorten the life of the cathode. In addition, since the intermediate layer 10 has a high resistance, it limits the current density possible to emit the electron by interfering flow of the electron emitting current.
In another aspect, a cathode for an electron gun comprising a metal layer mainly composed of tungsten of which the reducing degree is same as or smaller than silicone or magnesium and larger than nickel between the base metal and the electron emitting material layer, and the electron emitting material layer containing rare earth metal oxide to decompose the compound generated from the reaction, and thereby enabling the reducing element in the metal layer to serve to generate free radical barium atom is disclosed in Japanese patent laid-open No. 91-257735.
However, the cathode described above is stable at the beginning but its life cycle is suddenly deteriorated with the lapse of time since the additional reaction compound is generated when free radical barium atom is generated.